Liquid crystal display devices have been used for clocks, calculators, household electric appliances, measuring instruments, panels for automobiles, word processors, electronic organizers, printers, computers, televisions, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, an STN (super twisted nematic) mode, a DS (dynamic scattering) mode, a GH (guest-host) mode, an IPS (in-plane switching) mode, an OCB (optically compensated birefringence) mode, an ECB (electrically controlled birefringence) mode, a VA (vertical alignment) mode, a CSH (color super-homeotropic) mode, and FLC (ferroelectric liquid crystal). The driving method has been changed from conventional static driving to multiplex driving, and passive matrix driving and, recently, active matrix (AM) driving performed using, for example, TFTs (thin film transistors) and TFDs (thin film diodes) have become the predominantly used driving method.
As shown in FIG. 1, typical color liquid crystal display devices include two substrates (1) each having an alignment film (4), a transparent electrode layer (3a) serving as a common electrode and a color filter layer (2) disposed between one of the substrates and its alignment film, and a pixel electrode layer (3b) disposed between the other of the substrates and its alignment film. The substrates are disposed so that the alignment films face each other, and a liquid crystal layer (5) is sandwiched between the alignment films.
The color filter layer includes a black matrix and a color filter constituted by a red colored layer (R), a green colored layer (G), a blue colored layer (B), and optionally a yellow colored layer (Y).
A liquid crystal material constituting the liquid crystal layer has been highly controlled in terms of impurities because impurities left in the liquid crystal material considerably affect the electrical characteristics of a display device. It has been also known that, regarding a material of the alignment film, the alignment film directly contacts the liquid crystal layer and therefore impurities left in the alignment film moves to the liquid crystal layer, which affects the electrical characteristics of the liquid crystal layer. Thus, the characteristics of liquid crystal display devices resulting from impurities in the material of the alignment film are being studied.
Similarly to the alignment film material, a material such as an organic pigment used for the color filter layer is also assumed to affect the liquid crystal layer because of impurities contained in the material. However, it has been considered that, since there are an alignment film and a transparent electrode between the color filter layer and the liquid crystal layer, the direct influence of impurities on the liquid crystal layer is much smaller than the influence of impurities contained in the alignment film material. However, in general, the alignment film has a thickness of merely 0.1 μm or less, and even a common electrode which serves as a transparent electrode and is used on the color filter layer side and whose thickness is increased to increase the electrical conductivity has a thickness of merely 0.5 μm or less. Therefore, the color filter layer and the liquid crystal layer are not present in an environment in which they are completely isolated. The color filter layer may cause display defects such as white streaks, alignment unevenness, and image sticking because the voltage holding ratio (VHR) of the liquid crystal layer is decreased and the ion density (ID) is increased due to impurities contained in the color filter layer, the impurities affecting the liquid crystal layer through the alignment film and transparent electrode.
To overcome the display defects caused by impurities contained in a pigment constituting the color filter, the following methods have been studied: a method for controlling the elution of impurities that move into liquid crystal by using a pigment in which the proportion of an extract resulting from ethyl formate is adjusted to a particular value or less (PTL 1) and a method for controlling the elution of impurities that move into liquid crystal by specifying a pigment in a blue colored layer (PTL 2). However, these methods are not so different from a method in which the amount of impurities in a pigment is simply decreased. In a current situation in which a purification technique of pigments has advanced, these methods do not provide a sufficient improvement to overcome the display defects.
On the other hand, focusing on the relationship between a liquid crystal composition and organic impurities contained in a color filter, there are disclosed a method in which the difficulty of dissolving the organic impurities in the liquid crystal layer is expressed as a hydrophobic parameter of liquid crystal molecules in the liquid crystal layer, and the hydrophobic parameter is set to be equal to or more than a particular value; and a method in which, with consideration of a correlation between the hydrophobic parameter and an —OCF3 group at the liquid crystal molecule terminal, a liquid crystal composition containing, at a certain proportion or more, a liquid crystal compound having an —OCF3 group at the liquid crystal molecule terminal is prepared (PTL 3).
However, in the disclosure of PTL 3, the essence of the invention is also to suppress the influence of impurities in a pigment on the liquid crystal layer. The direct relationship between a structure of a liquid crystal material and a structure of a coloring material such as a dye/pigment used for a color filter has not been studied, which does not overcome the problems of display defects of highly advanced liquid crystal display devices.